1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to a method and apparatus for efficiently phase-shifting a plurality of signals and for sharing transmit shaping filters among the plurality of phase-shifted signals.
2. Description of Related Art
Transmit shaping filters are commonly used in communication systems in order to modify the baseband signal so that it is better suited for transmission over the intended channel. FIG. 1 is a symbolic block diagram of a transmit signal processing portion 10 for a wireless CDMA communication system now being developed in the IS-95B Standard. As shown, the IS-95B Standard presently contemplates the transmission of up to "n+1" digital baseband signals A.sub.0 -A.sub.n by directly sequencing the digital baseband signals A.sub.0 -A.sub.n with I and Q spreading codes to form in-phase I.sub.n and quadrature Q.sub.n signals; delaying the quadrature signals Q.sub.n by 1/2 a PN chip relative to the in-phase signals I.sub.n ; shaping the in-phase I.sub.n and quadrature Q.sub.n signals via "n+1" pairs of "baseband filters" 20 I, 20Q to produce filtered signals I.sub.n ', Q.sub.n '; modulating the filtered signals I.sub.n ', Q.sub.n ' with variously phase-shifted I and Q carriers ##EQU1##
to produce modulated components; combining the modulated components to form composite signals s.sub.0 (t) to s.sub.n (t), and combining the composite signals s.sub.0 (t) to s.sub.n (t)into a single transmit signal s(t).
The IS-95B Standard presently contemplates eight code channels (i.e. n=7), one fundamental channel transmitted without any phase offset and seven supplemental channels transmitted with the specific phase offsets shown in FIG. 2. Only four phase offsets, 0, .pi./4, .pi.n/2, and 3.pi./4 are needed for eight channels because each phase offset in the 0-to-.pi. range has a 180.degree. counterpart in the .pi.-to-2.pi. range. In other words, when transmitting information with in-phase and quadrature signals I, Q, transmitting I & Q with a phase offset of x is mathematically identical to transmitting -I & -Q with a phase offset of x.+-.180.degree..
If separate shaping filters are implemented for each channel as suggested by FIG. 1, then the hardware is subject to a great deal of complexity and increased power consumption. Separate transmit shaping filters are normally required, however, if the filtering hardware may not be fast enough to support multi-channel filtering operation.
The different phase offsets of the different channels also add design complexity, either in the analog domain or digital domain. As it is known, the input of the shaping filter for each channel in IS-95B is either 1 or -1. The design of separate filters is relatively simple, therefore, since no multiplication is involved. Implementing phase rotation before the filter or grouping multiple signals together, however, produces multiple signal levels at the filter input. Consequently, multiplication may be required, and the complexity of the filter design may significantly increase.
There remains a need, therefore, for a method and apparatus in a wireless communication system for simply and efficiently phase shifting, shaping, and modulating a plurality of inphase and quadrature signals derived from a plurality of digital baseband signals.